Window contamination on Expose-R

R. Demets1, M. Bertrand2, A. Bolkhovitinov1, K. Bryson3,4, C. Colas5, H. Cottin6,J. Dettmann1, P. Ehrenfreund7, A. Elsaesser7, E. Jaramillo8, M. Lebert9,G. van Papendrecht1, C. Pereira8, T. Rohr1, K. Saiagh6 and M. Schuster91ESTEC, Keplerlaan 1, 2201AZ, Noordwijk, The Netherlands e-mail: R.Demets@planet.nl2CNRS, CBM, UPR 4301, rue Charles Sadron, F-45071 Orléans, France3Bay Area Environmental Research Institute, 560 Third St West, Sonoma, CA 95476, USA4Space Science and Astrobiology Division, NASA Ames Research Center, Moffett Field, CA 94035, USA5Université Orléans, CNRS, ICOA, UMR 7311, rue de Chartres, F-45067 Orléans, France6Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), Université Paris Est – Créteil (UPEC), UniversitéParis Diderot (UPD), UMR 7583 CNRS, 61 Avenue du Général de Gaulle, 94010 Créteil, France7Leiden Observatory, Leiden University, Einsteinweg 55, 2333CC Leiden, The Netherlands8RUAG Schweiz AG, Schaffhauserstrasse 580, 8052 Zürich – Seebach, Switzerland9Lehrstuhl für Zellbiologie, Friedrich-Alexander-Universität, Staudtstraße 5, D-91058 Erlangen, Germany

Abstract: Expose is a multi-user instrument for astrobiological and astrochemical experiments in space.Installed at the outer surface of the International Space Station, it enables investigators to study the impact ofthe open space environment on biological and biochemical test samples. Two Expose missions have beencompleted so far, designated as Expose-E (Rabbow et al. 2012) and Expose-R (Rabbow et al. this issue). Oneof the space-unique environmental factors offered by Expose is full-spectrum, ultraviolet (UV)-richelectromagnetic radiation from the Sun. This paper describes and analyses how on Expose-R, access of thetest samples to Solar radiation degraded during space exposure in an unpredicted way. Several windows infront of the Sun-exposed test samples acquired a brown shade, resulting in a reduced transparency in visiblelight, UV and vacuum UV (VUV). Post-flight investigations revealed the discolouration to be caused by ahomogenous film of cross-linked organic polymers at the inside of the windows. The chemical signaturevaried per sample carrier. No such films were found on windows from sealed, pressurized compartments, oron windows that had been kept out of the Sun. This suggests that volatile compounds originating from theinterior of the Expose facility were cross-linked and photo-fixed by Solar irradiation at the rear side of thewindows. The origin of the volatiles was not fully identified; most probably there was a variety of sourcesinvolved including the biological test samples, adhesives, plastics and printed circuit boards. The outersurface of the windows (pointing into space) was chemically impacted as well, with a probable effect on thetransparency in VUV. The reported analysis of the window contamination on Expose-R is expected to helpthe interpretation of the scientific results and offers possibilities tomitigate this problem on futuremissions –in particular Expose-R2, the direct successor of Expose-R.

Received 1 April 2014, accepted 25 September 2014, first published online 18 November 2014

Key words: Solar UV, window transparency, contamination, outgassing, International Space Station.

Introduction

‘Expose’ is the short name of suitcase-sized scientific instru-ment developed by the European Space Agency (ESA) forthe conduction of astrobiological and astrochemical experi-ments in space. Installed at the outer surface of theInternational Space Station (ISS), it allows scientist to submittest samples to the harsh space conditions, comprising a com-bination of near-vacuum, full-spectrum Solar light, cosmicparticle radiation, near-weightlessness and wide-range tem-perature undulations. This complex environment can in itsentirety not be replicated in the laboratory on ground.Ideally, the test samples on Expose should be freely exposedto open space. In practice, however, chemical and biologicaltest samples on the ISS shall always be contained some wayor another to comply with the strict safety regulations ofmanned spaceflight. Therefore, all samples on Expose are

placed behind windows. Depending on the selected windowmaterial, Solar radiation can still reach the samples withouta significant reduction of ultraviolet (UV). The first spaceflightof Expose was completed in 2008–2009 under the designationExpose-E (Rabbow et al. 2012). This paper concerns its sistermodel, Expose-R, which was during 2009–2011 for 22 monthsin open space on the ISS (Rabbow et al. this issue). The top sur-face of Expose-R was equipped with seven large and 75 smallwindows, receiving Solar radiation at a total fluence of 17 GJm−2 during themission (Beuselinck &Van Bavinchove 2011b).Pictures made during extra-vehicular activity (EVA; space-walk) nr. 27 on the final day of exposure indicated that fourof the large windows had turned brown (Fig. 1). This was con-firmed 1 week later when Expose-R was back inside the ISS(Fig. 2). The four brownwindows were placed on top of samplecompartments, which had been connected to space vacuum

International Journal of Astrobiology 14 (1): 33–45 (2015)doi:10.1017/S1473550414000536 © Cambridge University Press 2014

during the exposure period, whereas the three non-affectedwindows belonged to pressurized, argon-filled compartments.Back on ground it was discovered that a similar discolourationwas displayed by many of the 75 small windows. As the browncolour should have impacted the quantity and quality of Solarlight that reached the test samples, affecting the core of thescientific goals, an investigation was started to identify theproperties and the root cause of the colour change. As such,this paper does not contain scientific results per sé. Instead, itis focused on technical investigations that were carried out tounderstand and possibly resolve this problem on future flights.

Materials and methods

After the spaceflight of Expose-R subsets of the windowsended up in various laboratories, depending on the experi-ments the windows were related to. In each laboratory, themeasurements were conducted using the available equipment.As a result, multiple spectrometers with overlapping featureshave been used, as listed below. Some of the windows weremeasured in more than one spectrometer at different institutes,with consistent results.UV/visible light (VIS) and vacuum UV (VUV)/UV spectra

were acquired to evaluate the loss of transmission over thesewavelength ranges. This was essential because the experimentson Expose-R were largely focused on the effects of Solar UVradiation. Fourier transform infrared spectroscopy (FTIR)spectra and X-ray photoelectron spectroscopy (XPS) spectrawere collected to identify the molecular structure of the con-taminants that had settled on the windows.

UV/VIS/NIR transmission spectroscopy (ESTEC)

UV/VIS/near-infrared spectroscopy (NIR) transmission spec-tra were made with a Cary 5000 spectrophotometer in dual-beam absolute specular transmission mode over a wavelengthrange of 190–2500 nm. Reference windows (blanks) and space-flown windows were measured sequentially.

VUV/UV transmission spectroscopy (LISA)

UV/VUV transmission spectra were acquired with a HoribaJobin Yvon spectrometer fitted with a monochromatorH20-UVL (grating: 1200 grooves per mm, dimensions 40× 45× 7 mm, opening of slits 130–160 μm) covering 100–300 nmwavelength range. Reference windows (blanks) and space-flown windows were measured sequentially. Spectra of theblanks were obtained by first measuring without a window.

UV/VIS transmission spectroscopy (NASA)

UV/VIS transmission spectra were collected with an OceanOptics HR4000 spectrometer. An Ocean OpticsDH-2000-S-DUV Deuterium Tungsten Halogen was used asspectral light source. The source and the collection fibre werealigned in one line. Blank MgF2 windows were used as refer-ence. Spectra were recorded in the 200–1100 nm wavelengthrange with an optical resolution of 0.91 nm (full-width at half-maximum (FWHM)).

UV/VIS transmission spectroscopy (RUAG)

UV/VIS transmission spectra were collected with a PerkingElmer Lambda 2 Photospectrometer, covering the 190–1100nm wavelength range. Reference windows (blanks) and space-flown windows were measured sequentially.

X-ray photoelectron spectroscopy (RUAG)

XPS analysis was performed using a Ph15000 VersaProbespectrometer (ULVAC-PHI, inc.) equipped with a 180o spheri-cal capacitor energy analyser and a detection system with 16channels.

Fourier transform infrared spectroscopy (ESTEC)

FTIR measurements were made on a Bruker Optics Equinox55 spectrometer. Spectra were acquired in the wavenumberrange of 4000–1100 cm−1 with 64 scans at a resolution of4 cm−1.

Fig. 1. The zenith-pointing top surface of Expose-R on the final day ofspace exposure. The picture was made outside the ISS during EVA 27and provided the first evidence for the discolouration of some of thewindows. Credit: NASA.

Fig. 2. Dmitriy Kondratyev with Expose-R inside the ISS when the 22month period of external exposure was over. The four brown-colouredwindows are easily discernable. Credit: Roscosmos.

34 R. Demets et al.

Fourier transform infrared spectroscopy (LISA)

FTIR measurements were made on a Bruker Vertex 70 spec-trometer. Spectra were acquired in the wavenumber range of4000–1000 cm−1 with a resolution of 4 cm−1. Each spectrumwas averaged on 32 scans.

Raman spectroscopy (ESTEC)

Raman spectroscopic analysis was performed with aRennishaw Invia Raman microscope, using the 514 nm laserexcitation with a laser power of 5 mW. Radius of the spot ofthe laser beam on the sample surface is approximately 1 μm.

Cleaning and polishing (LISA, NASA, RUAG)

To remove the contamination layer and to restore the trans-parency, the window surfaces were washed and/or polished.Acetone, methanol and dark soap (in water) manufacturedby Marius Fabre, France were used by LISA. An acetonewash was used byNASA. For polishing, fine diamond abrasivewas used by RUAG.

Determination of the thickness of the contamination layer(RUAG)

The windows were sputtered with argon ion etching at fivedepth points to find the composition at each depth. The thick-ness of the contamination was deduced from the results ob-tained in the five layers.

Handling and storage

All contaminated windows described in this paper werehandled and stored after the flight together with their non-contaminated counterparts. The nominal, full transmissionof the non-contaminated windows as measured after the flightproves that no (additional) contamination was introduced dur-ing post-flight handling and storage.

Results

Large windows

Loss of transparency – qualitatively

The brown colour was selectively displayed by a subset of win-dows (Figs. 1–3). No connection could be established with thematerials the windows were made of (Table 1). This indicatedthat the colour was somehow created from inside (a colouremerging outside, at the space side, should have appeared uni-formly over all windows). This preliminary conclusion wasreadily confirmed by visual inspection back on the Earth. Abrownish film was observed at the inner surface of windowsnrs. 8, 10, 5 and 23 (Fig. 3). A direct correlation could bemade with the atmospheric conditions behind the windows:the blurred windows were part of sample compartmentswhich, through venting lines, had been evacuated during theexposure period and were open to outer space during thewhole mission, whereas the clear windows belonged to sealedcompartments that had been kept under pressure (Table 1).This suggested that windows 8, 10, 5 and 23 were contaminatedby molecules that originated from the interior of their samplecompartments, considering that the release of volatile com-pounds is promoted by evacuation, but is suppressed by pres-surization (van Papendrecht et al. 2013).

Loss of transparency – quantitatively

For each window the reduction of the transparency was quan-tified. Transmission spectra were made by RUAG (200–1100nm) (RUAG 2011b) and the European Space Research &Technology Centre (ESTEC) (190–2500 nm) (vanPapendrecht et al. 2013) with consistent results. The trans-parency of the brown windows was not only reduced in the vis-ible range (400–770 nm) but even more strongly in UV(190–400 nm) (Fig. 4). Each window was measured fourtimes, covering different zones of the window surface (vanPapendrecht et al. 2013). Per window four identical curveswere acquired, indicating that the loss of transparency was

Fig. 3. Top surface of Expose-R on board of the ISS before (left) and after (right) space exposure. The numbers are identifiers for individualwindows. Nrs. 8, 10, 5 and 23 turned brown during exposure, while nrs. 6, 20 and 18 remained clear. Credit: NASA.

Window contamination on Expose-R 35

evenly distributed over the surface. Such homogeneity did notexist however among the four contaminated windows: eachwindow delivered its individual signature (Fig. 4). The remark-ably smooth curves, without ripples, peaks or dips (Fig. 4) didnot allow us to identify which molecular species were involvedand are indicative for cross-linked molecular aggregates.

Physical and chemical analysis of the brown film

The brown films were firmly attached to the windows andcould not be wiped away with a dry tissue. To investigate thephysical and chemical characteristics, the films were analysedby RUAG (RUAG 2011b) and ESTEC (van Papendrechtet al. 2013). The strongest loss of transparency was displayedby window no. 23 (Fig. 4). RUAG found that the film con-sisted of a solid, homogenous layer of 5–10 nm thickness.When that film was polished away using fine diamond abras-ive, the transparency was fully restored over all wavelengthsdown to 200 nm. XPS analysis byRUAGdemonstrated single-bond carbon (C–C) as the main contributor to this layer with aminor part played by carbonyl-bond carbon (C =O) (RUAG2011b). In a complementary FTIR analysis by ESTEC(van Papendrecht et al. 2013) signatures were found for aro-matic hydrocarbons, aliphatic hydrocarbons, carbonyl and

phenyl-methyl-silicone (Fig. 5). Between wavenumbers 1750and 1250 per cm a number of peaks were found (Fig. 5) thatcould not be linked to a specific molecular fingerprint. Suchpeaks however are typical for mixed, cross-linked molecules.As the window itself was made of pure MgF2, the brown filmwas clearly a deposit which had precipitated onto the windowduring the spaceflight.Like nr. 23, window nr. 5 was subjected to FTIR analysis by

ESTEC (van Papendrecht et al. 2013). Hydrocarbons and car-bonyl were once more identified but no clear signature for sili-con compounds showed up this time (Fig. 5). A possibleexplanation is that nr. 5 suffered from a comparable type ofcontamination as nr. 23, but to a lesser degree, which can beunderstood by the internal design of the carrier (Table 2).Whereas window nr. 23 was placed directly above the biologi-cal test samples, window nr. 5 was separated from the samplesby optical filters. These filters could have acted as a barrier forvolatile molecules to approach the window or as additionalsurfaces to settle upon – thereby reducing the net amount ofmolecules sticking to the window surface.Window nrs. 10 and 5 were placed on top of sample com-

partments with an identical internal lay-out and an identicalsample composition (Table 2). The transmission spectra ofnrs. 10 and 5 resembled one another, but were not fully ident-ical (Fig. 4). The small difference may be related to the windowmaterial, which was SiO2 for nr. 10 and MgF2 for nr. 5(Table 2). No FTIR profile was made of nr. 10, because SiO2

is opaque to infrared (IR).Although window nr. 8 suffered less than nrs. 23, 5 and 10 in

terms of transmission loss (Fig. 4), its brown film turned out tobe of a unique kind. Microscopy at high magnification byESTEC revealed a peculiar pattern of fractal-like structures(Butenko 2012). Raman spectroscopy by ESTEC indicatedthese structures to contain hydrocarbons, probably of a cross-linked kind. The true chemical identity was not establishedhowever although somematerial was removed from the surface

Table 1. Contamination of the Expose-R windows – qualitat-ively. The correlation with vacuum is evident

Windowcode

Windowmaterial

Browncolour

Compartmentcode

Internal atmos-phere of thecompartment

Nr. 8 SiO2 Yes 2–1 VacuumNr. 10 SiO2 Yes 2–2 VacuumNr. 5 MgF2 Yes 2–3 VacuumNr. 6 MgF2 No 2–4 Pressurized:

argon at 105 PaNr. 23 MgF2 Yes 3–2 VacuumNr. 20 MgF2 No 3–3 Pressurized:

argon at 105 PaNr. 18 SiO2 No 3–4 Pressurized:

argon at 105 Pa

Fig. 4. Transmission spectra of the large windows after spaceexposure. Vertical axis: transmission 0–100%, horizontal axis:wavelength 150–850 nm. Nr. 16 was a reference window, kept onground when Expose-R was in orbit.

Fig. 5. Post-flight FTIR spectra from windows nr. 23, nr. 5 and thenon-contaminated nr. 20. Suprasil quartz is opaque to IR hence noFTIR profiles could be made for windows nrs. 8 and 10. Vertical axis:transmittance from 88 to 100%. Horizontal axis: wavenumber per cmfrom 4000 down to 1100. Between 2000 and 1100 the axis has beenstretched to improve clarity.

36 R. Demets et al.

for further analysis (Butenko, 2012; van Papendrecht et al.2013). No FTIR profile wasmade of nr. 8, because SiO2 is opa-que to IR.

Small windows

Besides the seven large windows, the top surface of Expose-Rwas furnished with 75 small-sized circular windows from theexperiments IBMP, AMINO and ORGANIC (Fig. 6). Allwere made of MgF2. The SEEDS experiment was uniquelyequipped with a single square-shaped window (Fig. 6).

Small windows from IBMP

Ten out of the 75 small circular windows belonged to the IBMPexperiment (Fig. 6). The biological samples from IBMP camein three varieties: larvae from the midge, tomato seeds and rad-ish seeds (Fig. 7). Each sample was packed in a rectangular,UV-transparent, gas-permeable envelope made of polyolefin(Novikova et al. this issue), which was placed immediately be-hind the window (Fig. 7). Post-flight inspection revealed afaint, but clearly visible brownish circle at the inner surfaceof every window. The outer surface, pointing into space, wasalways clean (Fig. 8). Spectroscopy by ESTEC indicated thatthe transparency was reduced in VIS and UV (Fig. 9) in away comparable with large window nr. 8 (Fig. 4). The latterwindow was part of the IBMP experiment as well (seeTable 2). As the spectra from the ten small windows werenearly identical (Fig. 9) the transmission loss did not seem todepend on the biological species behind the window. As was

demonstrated by LISA, the transparency in VIS and UV wasfully restored when the brown circle was washed away withdark soap (in water), acetone and methanol (Fig. 10).The transmission spectra were expanded into the VUV by

LISA. The windows turned out to have largely lost their trans-parency at these smaller wavelengths (Fig. 11). Washing theinner surface using dark soap (in water), acetone and methanolhelped substantially to regain the transparency (Fig. 11).Whenin addition the outer surface was polished (washing of the outersurface had no significant effect) the transparency in VUV ap-proached the pre-flight percentages (Fig. 11).To explore the chemical identity of the brownish circle

(Fig. 8), eight small IBMP windows were subjected to FTIRanalysis by ESTEC. The FTIR profiles were the same foreach window, an example is shown in Fig. 12. The functional-ities most likely present were O–H, N–H, CH3, CH2, C =Oand Si-CH3.

Small windows from AMINO

Thirty small windows at the top surface of Expose-R belongedto the AMINO experiment (Fig. 6). The AMINO windowswere made of MgF2 and measured 11 mm in diameter, 1 mmin thickness. The samples of AMINO consisted of organic mo-lecules deposited as a thin layer at the inner surface of the win-dows. Thus, the side where the brown circle emerged in theIBMP experiment (Fig. 8) was in AMINO coated by a filmof chemical compounds as an integral part of the experiment.Contaminants settling at the inside of the AMINO windowscould therefore blend with the test samples and disturb thescientific objectives of this experiment. Two windows werekept free of samples. Post-flight analysis by LISA showed thatthese reference windows had largely lost their transparency inUV-C and VUV (Fig. 13) in a way reminiscent of the small win-dows from IBMP (Fig. 11). Washing the inner side with darksoap, water, acetone and methanol provided a substantial im-provement (Fig. 14). With both sides washed the transparencyalmost returned to pre-flight values in UV-C and near-VUV,but still not fully in far-VUV (Fig. 14). Polishing (cf. theIBMP experiment, Fig. 11) offered no further recovery fromthe transmission loss (data not shown).AMINO was equipped with dark controls which were pro-

tected against Solar light during space exposure. The dark con-trols contained two sample-free windows. After flight, both

Table 2. Internal configuration of the seven sample compartments equipped with a large window. Presence/absence of optical filtersand identity of biological materials. More details about the experiments in Rabbow et al., this issue

Window Window material

Optical filters

Biological materials (experiment short name)Neutral density filters Cut-off filters

Nr. 8 SiO2 No Yes IBMPNr. 10 SiO2 Yes Yes ENDO, OSMO, SPORES, PHOTO, SUBTILNr. 5 MgF2 Yes Yes ENDO, OSMO, SPORES, PHOTO, SUBTILNr. 6 MgF2 Yes Yes PURNr. 23 MgF2 No No ENDO, SPORESNr. 20 MgF2 Yes Yes ENDO, OSMO, SPORES, PHOTO, SUBTILNr. 18 SiO2 Yes Yes ENDO, OSMO, SPORES, PHOTO, SUBTIL

Fig. 6. Small windows on Expose-R and their attribution to theindividual experiments.

Window contamination on Expose-R 37

Fig. 8. Visual inspection of the small IBMP windows after the flight. A brownish circle was displayed at the inner surface of each window. The 7mm diameter of the circle corresponded to the Sun-exposed area (Fig. 7). The sketch is representative for all ten windows.

Fig. 10. Recovery from transmission loss of a small IBMP window inUV and VIS. (A) After space exposure. (B) After washing the innersurface. (C) After washing both surfaces. (D) After polishing the innersurface. (E) After polishing both surfaces. Vertical axis: transmission30–100%. Horizontal axis: wavelength 150–850 nm.

Fig. 9. Transmission spectra of ten small MgF2 windows from IBMPafter space exposure. Vertical axis: transmission 0–100%. Horizontalaxis: wavelength 150–850 nm. The spectrum after cleaning, samewavelength range is shown in Fig. 10.

Fig. 7. Close-ups of three samples from the IBMP experiment. Left: larvae of the midge. Centre: seeds of the radish plant. Right: seeds of thetomato plant. Each sample was wrapped in a heat-sealed polyolefin envelope, positioned behind a 1 mm thick MgF2 window. The windowsthemselves were placed behind 7 mm wide portholes.

38 R. Demets et al.

delivered the same transmission profile in UV-C/VUV as refer-ence windows that had been kept on ground (Fig. 13).To unravel the chemical identity of the washable

UV-blocker (Fig. 14), FTIR analysis was conducted byLISA on the sample-free windows. Two conspicuous groupsof FTIR peaks were found (Figs. 15 and 16), one between3000 and 2800 cm−1, the other below 1400 cm−1. Both clusterswere absent on reference windows that had been kept in dark-ness during the flight. Themolecular functionalities most likelypresent in the first group were CH3 and CH2, in the secondgroup Si–CH3 and Si–O–Si. Figure 16 shows a close-up ofthe first cluster. For reference, see Lee Smith (1960) andRomanenko & Tkachuk (1973).

Seeds

The single, rectangular window from the SEEDS experiment(Fig. 6) was not investigated after the flight.

Organic

The ORGANIC experiment occupied 35 positions at the topsurface of Expose-R2 (Fig. 6). Similar to AMINO, the samplesof ORGANIC consisted of chemical compounds deposited atthe inner surface of circular, 1 mm thick MgF2 windows. Twopositions were occupied by sample-free windows for reference.After the flight the transmission was measured by NASAAmes. A degradation was displayed (Fig. 17) comparablewith the small windows from IBMP (Figs. 9 and 10) andAMINO (Figs. 13 and 14). Like AMINO, ORGANIC wasequipped with dark controls, which never saw Solar radiationduring space exposure. In contrast to their Sun-exposed coun-terparts, the transparency of the dark controls was not affectedby the spaceflight (Fig. 17). The transparency of the affectedwindows turned practically back to normal when the innersurface was rinsed with acetone (Fig. 18).

Fig. 11. Recovery from transmission loss of a small IBMP window inVUVandUV-C. (A) After space exposure. (B) After washing the innersurface. (C) After washing both surfaces. (D) after polishing the innersurface. (E) After polishing both surfaces. (F) Before space exposure.Vertical axis: transmission 0–100%. Horizontal axis: wavelength 120–230 nm.

Fig. 12. Post-flight FTIR spectrum of a typical IBMP window.Functionalities most likely present: O–H, N–H, CH3, CH2, C =O andSi-CH3. Vertical axis: transmittance from 92 to 100%. Horizontal axis:wavenumber per cm from 4000 down to 1100. Between 2000 and 1100the axis has been expanded to improve clarity. Between wavenumber1750 and 1250 per cm undefined peaks were found that are typical formixed, cross-linked molecular aggregates.

Fig. 13. Transmission loss of sample-free MgF2 windows from theAMINO experiment. Wavelength range VUV and UV-C. (A,B)Non-flown references. (C,D) Flown in space, exposed to Solar light.(E,F) Flown in space but not exposed to Solar light. Vertical axis:transmission 0–100%, horizontal axis: wavelength 115–230 nm.

Fig. 14. Recovery from transmission loss of a sample-free MgF2

window from the AMINO experiment. Wavelength range VUV andUV-C. (A) Flown in space, exposed to Solar light. (B) After washingthe inner surface. (C) After washing both surfaces. (D) Before spaceexposure. Vertical axis: transmission 0–100%, horizontal axis:wavelength 115–230 nm.

Window contamination on Expose-R 39

R3D-R

Expose-R was equipped with an active sensor package calledR3D-R to record UV-C (170–280 nm), UV-B (280–315),UV-A (315–400 nm) and photosynthetically active radiation(PAR, 400–700 nm) over time (Dachev et al. this issue). Thethree UV photodiodes were each contained in a small cylindri-cal capsule (height 4.5 mm, diameter 5 mm) with a circularquartz window on top. The PAR photodiode was containedin a similar capsule topped by a transparent polyamide cap(Fig. 19). Since the capsules were fully closed, the inner surfaceof the three quartz windows and the polyamide cap was only incontact with the photodiode, whereas the outer surface was ex-posed to the space environment.Expose-R was in open space from 10 March 2009 till 21

January 2011. The weekly fluence measured by the R3D-Rfor UV-A, -B and -C and PAR is plotted in Fig. 20. Afterthe flight the three quartz windows were visibly clean. Thepolyamide cap however had turned from white to yellowish(Fig. 19). This discolouration resulted in a modest drop ofthe PAR signal over time (Fig. 20). The plots from Fig. 20

indicate that a slight degradation of transmission was probablyalso suffered by the three quartz windows in front of the photo-diodes. As the external space environment must have been thesame for all windows on Expose-R, this would suggest thatnone of windows on Expose-R were contaminated from out-side to the extent that the transparency in the UV range wasstrongly affected.Several Soyuz and Progress capsules have docked in close

vicinity of Expose-R during the 22 month exposure period(Table 3). During the final 15 min prior to docking, the thrus-ters from visiting vehicles are fired closely to the docking port(personal communication by astronaut Frank de Winne). In

Fig. 15. Post-flight FTIR spectrum of a Sun-exposed, sample-freeAMINO window after subtracting the readings from a dark control.Functionalities most likely present: CH3, CH2, Si-CH3 and Si–O–Si.Vertical axis: transmittance from 96 to 100%. Horizontal axis:wavenumber per cm from 4000 down to 1000.

Fig. 16. Close-up of FTIR peaks (range: 3000–2800 cm−1) displayedby a Sun-exposed, sample-free AMINO window. Functionalities mostlikely present: 1. CH3 anti-symmetric stretch, 2. CH2 anti-symmetricstretch for aliphatic chain, 3. Not known, 4. CH3 symmetric stretch foraliphatic chain, 5. CH2 symmetric stretch for aliphatic chain. Verticalaxis: transmittance from 98.6 to 100%. Horizontal axis: wavenumberper cm from 3000 down to 2800.

Fig. 17. Transmission loss of a sample-free, Sun-exposed MgF2

window from the ORGANIC experiment. (A) Ground reference. (B)Flown in space but not exposed to Solar light. (C) Flown in space,exposed to Solar light. Vertical axis: transmission 0–100%, horizontalaxis: wavelength 200–900 nm.

Fig. 18. Recovery from transmission loss of a sample-free,Sun-exposed MgF2 window from the ORGANIC experiment. (A)Ground reference. (B) Flown in space, exposed to Solar light, afterpost-flight washing of the inner surface. Transmission prior to cleaningis shown in Fig. 17. Vertical axis: transmission 0–100%, horizontalaxis: wavelength 200–900 nm.

40 R. Demets et al.

case exhaust products from thrusters were sprayed onto the topsurface of Expose-R during the docking events, this might haveresulted in contamination, even degradation, of the R3D-Rsensor windows with an ensuing drop in the output signal ofthe photodiodes. The high recording rate of the R3D-R (one

data point per 10 s) would even permit to detect contaminationdeposited temporarily on the surface of Expose-R. Temporarycontamination may occur when pollutants are first deposited,and next released by evaporation. To check for transient con-tamination, the output signals delivered by the four R3D-R

Fig. 19. The R3D-Rwas equipped with four photodiodes. Three were covered by a small circular quartz window in ametal ring. The fourth sensorwas covered by awhite-coloured polyamide disc which acquired a creamy colour during space exposure. Top row, left to right: pre-flight 14/8/2008,inside the ISS 7/3/2009, first day of exposure 10/3/2009. Bottom row, left to right: last day of exposure 21/1/2011, inside the ISS 28/1/2011,post-flight 23/3/2011.

Fig. 20. Mission fluence per week as recorded by the R3D-R from March 2009 till September 2010. Black: PAR, purple: UV-A, blue: UV-B,green: UV-C. The big swings over time are due to the changing attitude of the orbital plane of the ISS w.r.t. the Sun (=the beta angle). During 2009the solar recordings by the R3D-Rwere interrupted formanymonths due to a failure of an on-board computer. After September 2010 no datawereacquired. Horizontal axis: time (3 March 2009–4 September 2010). Vertical axis: fluence of UV-A, -B and -C in J m−2, PAR in arbitrary units.

Window contamination on Expose-R 41

sensors were investigated over the timeframes that visitors weredocking. The outcome of this investigation was negative; thesensors were not temporarily blinded during docking. An ex-ample is shown in Fig. 21.

Contamination sources

With the indications for hydrocarbons and Si-containing com-pounds provided by the FTIR spectra (Figs. 5, 12, 15 and 16)and Raman (Butenko 2012), an inventory was made of materi-als contained inside Expose-R that could possibly have deliv-ered the volatiles that were incorporated in the contaminationfilms on the windows.

Biological test samples

All biological test samples were desiccated before flight, but notoutgassed. As such, they must all be considered as potentialsources of contamination. The experiment-specific distributionof the biological samples over the sample compartments(Table 2) would partially explain why the chemical features ofthe contamination films were not identical in each compartment(Figs. 5, 12 and 15). However, the biological samples cannothave delivered the Si-containing compounds (Figs. 5, 12 and 15).

Chemical test samples

No biological samples were used by the experiments AMINOand ORGANIC. Instead, the test samples consisted of organicmolecules, deposited as a thin layer at the inner surface of the

windows. It is practically impossible that these chemical filmsproduced volatiles that were incorporated in the contaminationfilms detected on the sample-free windows (Figs. 13, 14, 17and 18). Also, there were no Si-containing compoundsamong the test samples of AMINO and ORGANIC. Thechemical samples can therefore not be held responsible forthe Si-peaks in Fig. 15.

Viton

A brownish colour was not only found at the inner surface ofthe windows but also on the Viton O-rings (RUAG 2011a).Used to seal the sample compartments towards the outside en-vironment, these O-rings were inserted between the windowsand the compartments. The brown colour on the O-rings wasconfined to the side facing the interior of the compartments.The question whether the O-rings had possibly produced con-taminants, which then propagated onto the windows, or theother way around, that the O-rings and windows were bothcontaminated from another source, was resolved by RUAGin favour of the latter option (RUAG 2011a). The brownlayer on the O-rings was definitely a deposition, not a degra-dation of the elastomer the O-rings were made of.

Wacker RTV-S 691 adhesive

RTV-S 691, based on silicone rubber, was used to glue the bio-logical samples into the sample carriers. It was present in allcompartments from Table 1. If nominally cured at room tem-perature, this adhesive has an outgassing potential ofCVCM= 0.07% (mass fraction that would still condense atroom temperature after outgassing in high vacuum) and anRML= 0.35 (mass fraction of molecular outgassing species ex-cept water). If post-cured at 65 °C – a procedure not followedduring the flight preparations because it would affect the bio-logical test samples – the outgassing potential can be reduced,but not fully eliminated (ESA-ESTEC 2004). It makes RTV-S691 a very likely source of silicone contamination. However,no RTV-S 691 was used in the sample compartments ofIBMP, AMINO, SEEDS and ORGANIC (Fig. 6). Still,Si-containing compounds were also here distinct contributorsto the contamination (Figs. 12 and 15).

Henkel Hysol EA 9361 adhesive

Hysol EA 9361, an epoxy paste adhesive, was used to fix theoptical filters (Table 2) into their frames. This glue was presentin all compartments from Table 1, the exception being com-partment 3–2 which was equipped with the heavily

Table 3. Docking events in vicinity of Expose-R during the period that R3D-R was acquiring data

Visiting vehicle Date and time (GMT) Docking port Position

Progress 33P 12/05/2009 19:24:23 DC-1/Pirs NadirProgress 36P 05/02/2010 04:25:59 SM/Zvezda (Service Module) AftProgress 37P 01/05/2010 18:30:21 DC-1/Pirs NadirProgress 38P 04/07/2010 16:17:00 SM/Zvezda (Service Module) AftSoyuz 20S/TMA-16 21/01/2010 10:03:17 MRM-2/Poisk (Mini Research Module) ZenithSoyuz 22S/TMA-18 04/04/2010 05:24:51 MRM-2/Poisk (Mini Research Module) Zenith

Fig. 21. Sensor output of the R3D-R during three orbits around thetime that Progress 33P was docking. Black vertical line: moment ofdocking (12 May 2009 at 19:24:23 GMT).

42 R. Demets et al.

contaminated window nr. 23. Curing of Hysol does not im-mediately pose a problem (as it does for Wacker RTV-S 691)because the filters can be glued into their frames in the absenceof the biological samples. Such was done when Expose-R wasprepared. The outgassing potential of this adhesive is similar toRTV S 691. Hysol does not release silicone species but mainlyC–H and C–O/C =O containing compounds.

3M Scotch-Weld adhesive

Scotch-Weld, an epoxy adhesive, was used to secure screwswhere no helicoils were present. The use of Scotch-Weld wasconfined to the compartments occupied by IBMP, AMINO,SEEDS and ORGANIC (Fig. 6). Epoxy outgassing productscontain mostly C–H and C–O/C =O chemistry.

Polymethylmethacrylaat

Each sample compartment from Table 1 contained a smallplastic box made of polymethylmethacrylat (PMMA), con-taining passive thermoluminescent detectors and polyallyldi-glycol carbonate radiation detectors. PMMA is a polymermade of hydrocarbons and ester derivatives.

Polyolefin

Polyolefin, a polymer made solely of hydrocarbons, was usedto contain the biological samples from IBMP (Fig. 7)(Novikova et al. this issue). Absorbing VUV light, any polymertends to decompose with formation of various gaseous pro-ducts, specific for the chosen polymer. Some of these productscould have taken part in the formation of the brown depositson the windows.

Electronics and heaters

The evacuated sample compartments were connected to the in-terior of the core facility of Expose-R where electronics andheaters were installed. If molecular traffic occurred from thecore facility into the sample compartments, a broad range ofmolecular contaminants may have been included (hydrocar-bons, silicones, esters, urethanes, etc.)

Discussion

Evidence was produced that during orbital flight the windowsat the zenith-pointing top surface of Expose-R lost a substan-tial part of their transparency, in particular in UV and VUV. Abrown-coloured contamination film was detected at the insideof the windows. The appearance of the film turned out to de-pend on two prerequisites: 1. Solar irradiation and 2. vacuumbehind the window. The latter was provided by a venting sys-tem, which connected the interior of the sample compartmentsto outer space.A comparable loss of window transmission caused by a

brownish deposit at the inside of the windows was reportedafter the flight of Expose-E in 2009. Indications were foundon Expose-E that a higher Solar fluence leads to a higherloss of window transmission (van Papendrecht et al. 2013).In a quantitative sense the contamination on Expose-E wasless prominent than on Expose-R, a difference that can be

linked to Solar irradiation: The top surface of Expose-R re-ceived a total fluence of 17 GJ m−2, for Expose-E it was6–12 GJ m−2 (Beuselinck & Van Bavinchove 2011a, b). Thequantitative connection between the intensity of the brown col-our and the fluence of Solar radiation (van Papendrecht et al.2013) is in agreement with the new finding from Expose-R thatreferencewindows, protected against Solar radiation but other-wise flown under the same conditions as the Sun-exposed win-dows, remained perfectly clean (Figs. 13 and 17).On Expose-E an -R, Sun-exposed windows placed on top

of pressurized compartments were not affected. To under-stand why, one has to keep in mind that the release of volatileorganic compounds is different in vacuum (outgassing) and atatmospheric pressure (offgassing). In vacuum, the release rateis much faster and a wider spectrum of volatiles can be emit-ted, including heavier molecules. Typically, under high vac-uum conditions the release rate becomes independent ofpressure and depends on matrix diffusion and activationenergy.The explanation provided above is compliant with most of

the observations, but not with all of them. In the MissionGround Reference (MGR, a replication of the flight exper-iment on ground, see Rabbow et al. this issue) there was nowindow contamination at all. To find out why, the test condi-tions in theMGRmust be compared with the situation in orbit.At least three factors were different in the MGR:Firstly, in theMGRSolar irradiation wasmimicked with the

SOL 2000, an instrument that provides UV from 200 nm andup. It means that the high-energy compound of the Solar UVspectrum was missing. Still, the cut-off at 200 nm does notcredibly explain why no windows were polluted in the MGR.The problem is the contamination of flight windows nrs. 8 and10 (Figs. 3 and 4, Table 1). Their inner surface never receivedUV< 200 nm because the window material was SiO2. It sug-gests that in principle, windows could get contaminatedunder the light regime offered by the SOL 2000.Secondly, there was no ‘core facility’ in the MGR. The

core facility is the suitcase-sized rectangular box, which ac-commodates the trays and sample carriers and provides themechanical and electrical interfaces with the ISS. Figure 2shows the trays and carriers installed in the core facility.Perhaps, the presence of the core facility was a pre-conditionfor the contamination to occur. If so, the core facility wouldbe a major provider of the volatile compounds from whichthe brown films are composed. This idea is not far-fetched.All sample compartments that were evacuated during theflight (=the ones with contaminated windows) wereequipped with venting lines to establish a connection to theinterior of the core facility; the latter was in direct contactwith the space environment by means of a series of smallopen holes. Molecules evaporating from the interior of thecore facility may have travelled through the venting linesto end up inside the sample compartments behind the win-dows. The core facility contained electronics and heaters,potential producers of volatile pollutants. Note that theevacuation of the sample compartments simply occurredby the opening of valves in open space, without a vacuum

Window contamination on Expose-R 43

pump. Contamination coming from a pump can therefore beexcluded.Thirdly, it is known that the outer surface of the ISS is not

only attacked by cosmic particles and atomic oxygen, but alsoby contaminants delivered from venting ports, thrusters, com-bustion products from visiting vehicles, etc. These chemicalspermanently surround the ISS as a cloud. That dirty cloudwas absent in the MGR. Still, in terms of physics it is noteasy to imagine how foreign molecules at extremely low press-ure can find their way first through venting holes into the corefacility and next through venting lines into the sample com-partments. The pressure outside the ISS is variable and fluctu-ates between 10−7 and 5× 10−4 Pa (Tighe et al. 2009). Inconclusion, if the external atmosphere of the ISS played arole in the transmission loss of the Expose-R windows, itshould have been primarily at the outer window surface, notthe inner surface.The outer surface of the windows has so far been left out of

this discussion. Before Expose-R was moved out of the ISS forexternal exposure, an incident happened whereby a crewmember touched with his bare skin the small MgF2 windowsof ORGANIC. This could have led to a reduced transparencyof these windows. No verification was possible however afterflight, because when the 22 month exposure period was overthe outer surfaces of Expose-R including the windows werewiped clean by the crew using tissues wetted with 3% H2O2.This cleaning process was part of a standard procedure, theobjective was to remove any toxic products (originatingfrom the dirty cloud, see above) from equipment that hadstayed outside. Figure 1 taken after the exposure period butbefore the cleaning had begun, shows that the darkeningwas confined to windows nrs. 23, 5, 10 and 8 (Fig. 4). Itmeans that any external deposit removed by the crew musthave been colourless. If there were a colourless, wipeable de-posit at the outer surface of the windows, this still could haveacted as a UV-blocker. In that case, only UV at wavelengths<170 nm should have been blocked because the UV-C sensorof the R3D-R (sensitivity: 170–280 nm) appeared to deliver asignal at almost full strength throughout the mission (Fig. 20).Note that the recordings from Fig. 20 were made before thecrew started to wipe Expose-R clean.There is direct evidence that the outer surface of the windows

was not so much polluted during space exposure, but stillunderwent a chemical attack. XPS analysis byRUAG revealeda 20 nm thick corrosion layer at the outer surface ofMgF2 win-dow nr. 23 with Si, C and O as the main contributors (RUAG2011b). Evidently, that layer could not be removed withH2O2-impregnated wipes. Although more than twice as thickas the brown-coloured film at the inner surface, the outerlayer turned out to be fully transparent over all wavelengths>200 nm (RUAG 2011b). Considering that the externalspace environment must have been the same for all windows,a corrosion layer as reported for window nr. 23 was probablypresent at the outer surface of all MgF2 (and possibly alsoSiO2) windows. If such were the case, then the affected outerlayer contributed not (or only little, see Fig. 18) to the re-duction of transmission in VIS and UV. Figures 11 and 14

however suggest that flight-induced changes at the outer sur-face may have contributed to the reduction of window trans-mission in the VUV range (<200 nm).In summary, the main culprit for the transmission loss ap-

pears to be the layer deposited at the inside of the windows,built from volatile compounds originating from the Expose-Rfacility and its contents (‘self-contamination’). Photo degra-dation and fixation by Solar light must have turned these com-pounds into macromolecular, polymeric films firmly attachedto the window surface. Compartment-specific differences werefound in the transmission curves (Fig. 4) and FTIR profiles(Figs. 5, 12 and 15) of these films. This suggests that the pollu-tants were, at least partly, delivered by the test samples them-selves, because the sample composition was different percompartment.The FTIR technique is a see-through method which, in

principle, does not tell if the observed features pertain tothe inner or outer surface of the investigated window. Still,the obvious differences in FTIR signatures (Figs. 5, 12 and15) can only be attributed to the inner window surfaces, be-cause the outside of all windows was exposed to an identicalenvironment during the spaceflight. The FTIR spectra wereintended to identify the chemical compounds involved inthe contamination layer qualitatively, not quantitatively.The latter is in principle possible as well using FTIR, but re-quires additional calibration procedures plus knowledgeabout the nature of the contaminants, which was – and stillis – not fully available.Non-biological parts in the sample compartments, in par-

ticular the adhesives, appear to have played a role as well.An important contribution from the core facility cannot beruled out, as explained above. New, preliminary data obtainedby mass spectroscopy at CNRS in Orléans hint at the possibleinvolvement of polyethylene glycol (PEG), polypropylene gly-col (PPG) and phthalates. PEG and PPG are very commonsynthetic plastics, phthalates are substances added to plasticsto increase their flexibility, transparency, durability and/or lon-gevity. These compounds have CH3, CH2 and C =O function-alities, which are in accordance with the FTIR spectra.

Conclusions

A next Expose mission has currently been started. CalledExpose-R2, a new set of trays, carriers and samples has beenuploaded to the ISS. The core facility will be the same onethat served on Expose-R. Therefore, Expose-R2 will utilize acore facility that has been seasoned for 22 months in near-vacuum. It means that the outgassing potential should be mini-mal under the thermal operational environment. That wouldleave the fresh samples and the glues as the main producersof contaminants on Expose-R2. This obstacle is planned tobe tackled by a new operational procedure in orbit. After in-stallation outside the ISS, the windows will not immediatelybe exposed to the Sun (as was the case for Expose-R).Instead, a protecting hood will remain in place for minimally6 weeks. Throughout that period, the venting lines will stayopen to allow evacuation. Expectedly, the samples and the

44 R. Demets et al.

glues can release part of their volatile compounds without anensuing photofixation at the window surface. Instead, it ishoped that the vaporized molecules will exit from the samplecompartments through the venting lines. When later on thehood is removed to expose the samples to Solar light, the alreadyoutgassed volatile compounds should have disappeared, mini-mizing the potential for continual photofixation processes.

REFERENCES

Note: Several of the references below are not scientific publications buttechnical documents that were published to support the Expose project.They can be requested from the corresponding author.

Beuselinck, T. & Van Bavinchove, C. (2011a). Expose: Environmentalhistory by calculation: Expose-E Simulation Results. RedShift Designand Engineering BVBA, EXP-RP-017-RS, Issue A, Rev. 2, 6 May2011.

Beuselinck, T. & Van Bavinchove, C. (2011b). Expose: Environmental historyby calculation. Expose-R simulation results. RedShift Design andEngineering BVBA, EXP-RP-020-RS, Issue A, Rev. 1, 23 December 2011.

Butenko, Y. (2012). Raman spectroscopy and optical post-flight inspection ofthe window Suprasil 8, Expose R. Materials Report nr. 6754. ESTEC(TEC-QTE), 20 January 2012.

Dachev, T., Horneck, G., Häder, D-P., Schuster, M. & Lebert, M. (2014).EXPOSE-R Cosmic Radiation Time Profile. Int. J. Astrobiol. (this issue),published online: 12 May 2014, Doi: http://dx.doi.org/10.1017/S1473550414000093

ESA-ESTEC (2004). RTV-S 691. In Space product Assurance. Data forselection of space materials and processes. ECSS-Q-70–71A Rev. 1,p. 142. ESA Publication Division, 18 June 2004.

Lee Smith, A. (1960). Infrared spectra-structure correlations fororganosilicon compounds. Spectrochim. Acta 16(1–2), 87–105.

Novikova, N., Deshevaya, E., Levinskikh, M., Polikarpov, N., Poddubko, S.& Sychev, V. (2014). Study of the effects of the space environment ondormant forms of biological specimens. Int. J. Astrobiol. (this issue).

Rabbow, E. et al. (2012). EXPOSE-E: an ESA Astrobiology Mission 1.5Years in Space. Astrobiology 2(5), 374–386.

Rabbow, E. et al. (2014). The Astrobiological Mission EXPOSE-R on boardof the International Space Station. Int. J. Astrobiol. (this issue), publishedonline: 28 August 2014, doi: http://dx.doi.org/10.1017/S1473550414000202

Romanenko, E.A. & Tkachuk, B.V. (1973). Infrared spectra and structure ofthin polydimethylsiloxane films. J. Appl. Spectrosc. 18, 188–192.

RUAG (2011a). EXPOSE-R. Post-flight De-integration and Inspection,RUAG EXR-TN-HT-017, Issue 1, 14 April 2011.

RUAG (2011b). EXPOSE-R. Post-flight Inspection of Windows, RUAGEXR-TR-HT-010, Issue 2, 21 July 2011.

Tighe, A.P., Iwanovsky, B., van Eesbeek, M. & Duzellier, S. (2009). In-orbitmeasurements of the Columbus lab vacuum environment using theMEDET pressure gauge. In ISMSE-11, Int. Symp. on Materials in SpaceEnvironment, Aix-en-Provence, France, September 2009.

van Papendrecht, G., Butenko, Y., Bolkhovitinov, A., Demets, R., Rohr, T.& Semprimoschnig, C. (2013). Molecular contamination effects on thesample compartments of the Expose-E and -R flight experiments(ISMSE-12). In ESTEC (TEC-QTE), Proc. 12th Int. Symp. onMaterials in Space Environment, Noordwijk, The Netherlands. ESASP-705, February 2013.

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